Stereo image processing apparatuses which measure a distance to a target have been known heretofore. Such a stereo image processing apparatus (hereinafter simply referred to as “stereo image processing apparatus”) extracts, from a reference image, a corresponding point to a target image. Here, the “corresponding point” refers to a point at which an object identical to an object captured at a target point in a target image. The stereo image processing apparatus calculates a distance to an object based on a camera parameter and a disparity which represents a shift amount of a corresponding point relative to the target point.
Applications of stereo image processing apparatuses in consideration include a safety apparatus that measures a distance to a forward vehicle or a pedestrian captured by a vehicle-mounted camera. Such safety apparatuses are about to widespread also in small-sized automobiles in recent years. For this reason, there is a demand for miniaturization of stereo cameras used for stereo image processing apparatuses.
As the size of stereo camera decreases, the distance between a camera that captures a target image and a camera that captures a reference image decreases and a disparity decreases. For this reason, the stereo image processing apparatus is required to perform highly accurate disparity calculation.
A conventional technique disclosed in PTL 1 is an example of technology for highly accurate disparity calculation. According to this conventional technique, a disparity is calculated using one-dimensional POC (Phase Only Correlation) scheme.
More specifically, a one-dimensional pixel data sequence is clipped using a Hanning window from each of the target image and the reference image, and the clipped pixel data sequences are synthesized after performing one-dimensional Fourier transform. The related art normalizes the amplitude component of the synthesized data sequence and calculates a phase-only correlation coefficient by one-dimensional inverse Fourier transform. In addition, the related art calculates a disparity based on the correlation peak of the calculated phase-only correlation coefficient.